1. Field of the Invention
The present invention relates to a spectral ellipsometer for observing changes in the polarized conditions of light reflected off a surface of a test substance to permit the measuring of optical constants, such as index of refraction, and the extinction coefficient of a sample. When a thin film layer is present on the surface of the test substance, measurements of film thickness and optical constants can also be achieved, and more particularly, the provision of improved output signals that address chromatic aberrations.
2. Description of Related Art
Referring to the measurement system shown in FIG. 4, a directed light beam from a source of light 2 is folded in an optical system 4 and directed to a polarizing prism 5 to output a linearly polarized beam of light 6 to contact the surface 8a of a sample 8 on a sample station 7. The sample 8 can have a surface with a thin film and the polarized conditions of the reflected light beam 10 will change, depending on the thickness of the thin film, the index refraction and extinction coefficient of the thin film. This change in the polarized condition of the testing light beam is due to the fact that a difference in reflected light exists between P polarization and S polarization, depending on the respective shifts in phase of reflection and the reflection coefficient. As a result, it is possible to obtain the thickness of the thin film or the index of refraction of the sample surface 8a upon measuring the amount of change in polarization of the reflected light and performing certain pre-determined analytical calculations. The reflected light beam 10 can be subject to a modulator 12 and analyzer 13 and a folding detecting optical system 9 for directing the reflected light to an optical fiber 14 to input signals to the spectroscope 11.
As the miniaturization of semiconductor products advances, there are additional requirements to evaluate the film qualities of ultra-thin films in numerous fields, including the manufacturing of semiconductor components. In the semiconductor field, gate oxide films and low absorption films have been increasingly thinner in the new generation of semiconductor devices. Additionally, there are requirements to accurately evaluate multi-layered structures that are employed in flash memories wherein oxide films and nitride films of silicon are alternately laminated. Additionally, multi-layered films are also formed on SOI wafers and the like.
This industry is also requiring light measuring instruments, such as spectral ellipsometers, to be capable of performing highly accurate measurements over a wide range of wavelengths ranging from ultraviolet to visible to infrared radiation. Such instruments must be further capable of performing measurements of the dependency of thin film properties on such variable wavelengths. In the conventional spectral ellipsometers that are provided with a polarizer for changing the light of a wide range of wavelengths into inear polarization, usually a prism will serve as a polarizer since it may be used in a wide range of wavelength regions from visible light to the ultraviolet region. Additionally, the prism assists in the optical quenching ratio, transmittance and similar features.
Generally, such a prism has a thickness ranging from approximately 10 mm to several tens of mm. As such, it is not possible to focus the optical axes of a wide variety of wavelength regions onto one spot, owing to differences, in, for instance, the indices of refraction with respect to wavelengths of light when reducing a beam diameter. As a result, a drawback of increased chromatic aberrations, such as shown in FIG. 5, can occur wherein a depth or zone of focus will result, such that a focusing position of Q2 of a longer wavelength will have its focus position extending further when compared to a focusing position on the optical axis of Q1 of a light having a shorter wavelength. Thus, in this context, the term xe2x80x9cchromatic aberrationxe2x80x9d indicates differences in indices of refraction only to wavelengths, that is, aberrations resulting from dispersion (defects caused in cases where an image forming system does not satisfy conditions for Gauss image formation).
There is still demand in the prior art to address these issues as requirements of greater accuracy is being imposed upon measuring instruments such as spectral ellipsometers.
The present invention addresses the issues of chromatic aberrations by providing a spectral ellipsometer in which it is possible to easily focus all the optical axes of multiple wavelengths onto one spot by means of a relatively inexpensive improvement.
The spectral ellipsometer of the present invention includes a source of light and an optical system for directing light of multiple wavelengths to an optical element for receiving the multiple wavelength light and focusing the multiple wavelength light onto a single spot on a sample surface. A detecting optical system receives the multiple wavelength light after contact with a sample surface and it is able to process it to an output signal providing the desired information. The detecting optical system can accurately receive and transmit any change in polarization in the elliptical polarization light reflected by the sample surface. The optical element that is placed in the light incidence optical system can be a prism with a particular shape for a light incidence surface and a light exit surface, both of which can be curved so that the light is orthogonal with respect to a progressing direction of the respective directions of incident and outgoing light thereby providing a spherical prism whose curved surfaces address the range of wavelengths that are incident upon the spherical prism.
The present invention provides not only an improved spectral ellipsometer, but also a method of ensuring the focusing of light of multiple wavelengths onto a single spot on the sample surface. The light incident surface and light exiting surface of the spherical prism will, in a macroscopic sense, be orthogonal with respect to all the optical axes of the incident wavelength light so that it will be possible to substantially eliminate the phenomena of refraction of incident light and to focus across a wide range of wavelengths, all ray traces onto one spot on the sample surface to thereby prevent occurrence of chromatic aberrations.